Various proposals have been made for generation of effective "anti-noise" signals and reference may be made to the specifications of U.S. Pat. Nos. 4122303 and 4153815.
This invention is concerned with improvements in a simple system for active noise cancellation which operates in the frequency domain and is sometimes referred to as the "virtual earth" system. This system is described for instance in the specification of U.S. Pat. No. 2983790 (Olson). The "virtual earth" system can be used to create a quiet zone in the vicinity of a microphone disposed in a sound field, by locating a loudspeaker closely adjacent to the microphone (e.g. some 10 cms away) and coupling the microphone and loudspeaker into a loop circuit producing an overall gain greater than unity and a 180.degree. phase reversal. This known "virtual earth" system operates by continually controlling the output from the loudspeaker so that it nulls the sound field at the microphone.
The known arrangement is shown in the first figure of the accompanying drawings to be discussed hereafter from the discussion of that figure, the limitations of the known system will become evident.
The present invention seeks to increase the distance over which a "virtual earth" system is effective without reducing the frequency range over which the "virtual earth" system can operate.